1. Field of the Invention
The invention relates to an infinitely variable transmission (IVT) which establishes a torque circulation in combination with a planetary gear, and an infinitely variable transmission using a belt type continuously variable transmission and, more particularly, to an infinitely variable transmission for establishing a neutral state by the torque circulation.
2. Description of Related Art
Recently, in order to improve fuel consumption rate and drivability, an automatic transmission has been used in which a belt type continuously variable transmission (CVT) is mounted as the transmission of a vehicle.
Conventionally, as disclosed in Japanese Patent Laid-open No.6-331000, an infinitely variable transmission is known which includes a continuously variable transmission, a constant speed mechanism and a planetary gear mechanism so that a shift range may be maximized by synthesizing the power from the planetary gear mechanism and the constant speed mechanism in the planetary gear mechanism, in order to establish a power (or torque) circulation in the continuously variable transmission. In this infinitely variable transmission, the engine output is transmitted through the constant speed mechanism to a carrier and further through the continuously variable transmission and a first (low) clutch or a one-way clutch to a sun gear. In this state, as the torque circulation occurs in the continuously variable transmission so that its gear ratio increases from a low (O/D) to a large (U/D), the gear ratio of the output shaft of the infinitely variable transmission from a ring gear changes in the direction of backward .fwdarw..infin. (at the output RPM of 0).fwdarw.forward high (U/D).fwdarw.forward low (O/D). When the first clutch or the one-way clutch is released such that a second (high) clutch is applied, the rotation from the continuously variable transmission is transmitted directly to the output shaft so that the rotational ratio of the output shaft is shifted from high (U/D) to low (O/D) as the gear ratio of the continuously variable transmission changes from high (U/D) to low (O/D).
In the infinitely variable transmission establishing the torque circulation, by setting the gear ratio of the continuously variable transmission to the value predetermined by the gear ratio of the planetary gear mechanism, a gear neutral position exists in which the RPM of the output shaft is geometrically at 0. As a result, the infinitely variable transmission is theoretically constructed without any starter unit such as a starter clutch or a torque converter.
In the infinitely variable transmission described above, however, even if the gear neutral position theoretically exists, the output torque diverges to infinity in the vicinity of the neutral position. As a result, the output torque changes a lot when the continuously variable transmission deviates slightly from the predetermined gear ratio for the neutral point. Actually, it is difficult to hydraulically control the axial forces to act on the two pulleys of the belt type continuously variable transmission to set/hold the pulley ratio at the target gear ratio for the neutral position.
Therefore, the industry lacks an infinitely variable transmission (IVT), in which the belt type continuously variable transmission is automatically converged to the gear neutral position by equalizing or confining within a predetermined range the axial forces to act on two primary and secondary pulleys.
In such an infinitely variable transmission, the torque ratio diverges to infinity in the gear neutral position. It takes an extremely large value in the vicinity of the neutral position. Thus, the input torque to the infinitely variable transmission has to be limited because it is regulated by the allowable transmission torque capacity of a transmission element of the infinitely variable transmission, especially the belt of the CVT.
Furthermore the industry lacks an infinitely variable transmission which generally employs an internal combustion engine as its prime mover. In the vicinity of the gear neutral position (at a vehicle starting time) in which the torque ratio takes the maximum, the throttle opening is suppressed to an extremely small value (e.g., about 5%) by the limit of the input torque. Even the engine output by the extremely small opening throttle can satisfy the necessary torque because the infinitely variable transmission takes the maximum torque ratio. In the extremely low opening of the throttle, however, an equi-throttle curve of the engine output characteristics has a tendency to go down rightwards so that the engine RPM drops a lot. As a result, even when a driver depresses the accelerator pedal to accelerate the vehicle, a sufficient torque is outputted, but the engine RPM remains at a low level. When the CVT is up-shifted (or shifted in the U/D direction) to raise the engine RPM, however, sufficient torque cannot be outputted.
These characteristics provide a physical disorder to the driver because they are different from those of the characteristics of a starter unit such as a conventional torque converter. Specifically, as the accelerator pedal is depressed, the engine RPM is raised accordingly, and the input torque of the transmission is increased.